1. Field of the Invention
This invention generally relates to a variable valve timing structure, and more particularly relates to a variable valve timing structure for an outboard motor.
2. Description of Related Art
A typical outboard motor comprises a power head and a housing unit depending from the power head. The power head includes an internal combustion engine that drives a marine propulsion device (e.g., a propeller) through a driveshaft and a propulsion shaft, which are both journaled within the housing unit. The marine propulsion device is attached to the end of a propulsion unit, which extends from housing unit and is in a submerged position.
There is an increasing emphasis on obtaining more effective emission control, better fuel economy and, at the same time, continued high or higher power output in outboard motors. Accordingly, four-cycle engines have started to replace two-cycle engines in outboard motors. However, it is difficult to arrange all the components of a four-cycle engine into the limited of space of an outboard motor cowling.
It is also desirable to achieve good emission control, fuel economy and high power output during the entire speed and load range of the outboard motor. In automotive four-cycle engines, there have been proposed a wide variety of devices to permit the engine characteristics to be adjusted during operation to obtain optimum performance across the entire speed and load range. One such device is a variable valve actuating mechanism, which includes both changing valve timing and/or the valve lift. The valve timing usually is advanced in the high engine speed range to effect higher charging efficiency and higher performance. At lower engine speeds, the timing typically is delayed to effect higher combustion efficiency, fuel economy and good emission control.
Typically, such variable valve actuating mechanisms are hydraulically operated. The working fluid for operating the mechanism is typically provided by the lubrication system of the motor. The pressure of the working fluid is used to actuate various parts of the variable valve actuating mechanism.